Sharpen.FilePath constructor and java.io.File behave differently

[linquize]

Current implementation of Sharpen.FilePath constructor and java.io.File behave differently. Especially in Windows.
"C:", "\ABC\DEF" combines the wrong path

We may refer to the java test to match the behaviour
http://hg.openjdk.java.net/build-infra/jdk7/jdk/raw-file/00cd9dc3c2b5/test/java/io/File/Cons.java

but we also need to consider backward slash "" which is not covered by java test

[alanmcgovern]

I'll add this case to the testsuite and try to work up a fix. I'm not sure if I can look at that java file as it is probably licensed under the GPL or LGPL whereas MonoDevelop requires all code to be under MIT/X11 or Apache 2. The best way for me to fix issues with this code would be to submit testcases so I can re-implement the required behaviour.